This invention pertains to the use of J-tubes on offshore platforms for installing pipelines extending upwardly from the seafloor.
In offshore petroleum operations, a platform frequently serves as an operational base and a production terminal for oil and gas wells. A problem common to all such offshore platforms is providing a connection from an above-water facility to a pipeline which lies on the seafloor. A typical method of providing the connection is the use of a J-tube riser guide member which is a continuous tubular structure having the shape of a J and through which a riser pipeline is pulled from the seafloor to the above-water facility. This J-tube runs down along the length of the platform, bends near the seafloor and thus runs generally parallel to the seafloor a short distance. The lower end of the J-tube frequently includes a flared portion referred to as a bell mouth. The upwardly pulled pipeline, i.e., the riser, provides a fluid flow path from the above-water facility to the pipeline on the seafloor.
At least two basic methods are known for installing the pipeline riser in the J-tube once the J-tube is positioned. In the so-called J-tube method, the riser is inserted in the seafloor end of the tube and pulled through it with a cable or other means. In the so-called reverse J-tube method, a section of the riser is inserted in the deck end of the tube and is pulled or pushed down through it as additional sections of conduit are joined to the riser at the deck. Both of these methods place stresses on the riser as it is forced through the J-tube. These stresses may cause displacement to the riser and J-tube which produce strains that exceed the limits of the materials and cause damage to the riser or J-tube.
Unfortunately, the J-tube is generally useful for riser pipeline diameters only up to about 12 inches. For riser pipelines having a diameter greater than 12 inches, the high pulling force required to install the riser is a major deterrent to further use of the J-tube. The J-tube has the advantage of a controlled and simple installation process which does not require elaborate pipeline manipulating equipment, unlike other known installation processes.
Accordingly, it is a purpose of the present invention to provide a J-tube which has all the advantages of the conventional J-tube and which can be utilized with pipeline diameters greater than 12 inches without placing undue stresses upon the pipeline or the J-tube.
Applicant is not aware of any prior art references which, in his judgment as one skilled in the pipeline art, would anticipate or render obvious the instant invention; however, for the purposes of developing the background of the invention and establishing the state of the requisite art, the following are set forth: U.S. Pat. Nos. 4,098,091, 4,523,877, 4,106,063 and 4,039,717.